ORDERS OF FISHES, 237 
ments which are richly supplied with blood, and are believed 
to discharge in part the function of gills. The mouth (fig. 127, 
m) opens into a dilated chamber, which is believed to represent 
the pharynx, and is termed the pharyngeal or “ branchial” sac. 
The walls of this chamber (/) are strengthened by numerous 
- cartilaginous filaments, between which are a series of transverse 
slits or clefts, and the whole is covered with a richly ciliated 
mucous membrane. The function of this sac is clearly respira- 
tory, the water from without being admitted through the mouth, 
passing through the branchial clefts into the abdominal cavity, 
and finally escaping by means of an aperture placed on the 
ventral surface a little in front of the anus. From the hinder 
end of the branchial sac proceeds the alimentary canal, which 
has appended to it a sac-like organ believed to represent the 
liver, and which terminates behind in a distinct anal aperture. 
Fig. 127.—Diagram of the Lancelet (Amphioxus lanceolatus): mt Mouth with 
cartilaginous filaments; # Greatly developed pharynx, or branchial sac, per- 
forated by ciliated apertures ; z Intestine; 2 Anus ; % Blood-vessels, with pulsat- 
ing dilatations in place of a heart ; ch Notochord ; z Spinal cord. 
There is no heart, and the circulation is entirely effected by 
means of several contractile dilatations, developed upon the 
great blood-vessels (Z). The blood itself is colourless. No 
kidneys have hitherto been discovered, and the reproductive 
elements are emitted into the abdominal cavity, from which 
they escape by the pore placed upon the lower surface. 
There is no skeleton properly so called. The notochord (ch) 
remains throughout life as a semi-gelatinous rod, enclosed in a 
membranous sheath, and supporting the spinal cord. There 
is no skull, and the spinal cord (z) does not expand in front to 
form a distinct brain or cerebrum. The brain, however, may 
be said to be represented, as the front portion of the nervous 
axis gives off nerves to a pair of eyes, and another branch to a 
